Seabed geochemical indications of migrated hydrocarbons provide critical information that mature source rock and effective migration are present. Hydrocarbon seepage can, in selected geological settings, also delineate subsurface petroleum accumulations and provide information on hydrocarbon charge and type. Surface geochemistry has been used for many years with varied success, attesting to the complex relationship between the surface and subsurface. Not all petroleum-bearing basins have a detectable surface signal. In addition, not all petroleum accumulations have a vertical, or nearly vertical, geochemical signature with the surface. Understanding controls on hydrocarbon movement from depth to surface, as well as near-surface processes that can alter, mask or confuse the surface geochemical signal, are critical for effective use of seabed seepage data in exploration.